I find the joy of sandboxing is that my players create the adventures with me, by deciding where to go & what to do. Of course there needs to be some pre-created content to give them a grounding for action, but it's a lot easier to create material in response to player decisions than to create a plotted adventure from nothing. Eg currently my PCs addressed the Altanian clan moot and decided to aid them vs Neo-Nerath by embarking on a lengthy overland quest to reach the Gate Castle of the Black Sun and close the Gate that is the source of Neo-Nerath's vast Necromantic powers. They also decided on their route, from several possible. All I then have to do is detail the quest site (pretty easy when I have Dyson's Delves I & II, I soon found useable maps). Much of the overland journey is created through encounter tables with encounters rolled during play (last session a CR 17 dragon turtle attacked their ship!), in conjunction with the pre-existing set-piece locale notes.
Read more: http://www.enworld.org/forum/showthread.php?472691-Collaborative-adventure-creation/page2#ixzz3rjobvoUt